Two raters independently assessed seizure duration in single-channel electroencephalogram (EEG) records obtained in 479 electroconvulsive treatments of 52 patients. Ratings of seizure duration were highly reliable. The measures of EEG seizure duration were substantially associated with the duration of motor manifestations, supporting the validity of the EEG determinations. Confidence ratings of the clarity of the endpoint of EEG paroxysmal activity were unrelated to the duration of seizures in EEG and motor measures. The extent to which the EEG seizure activity extended beyond the motor manifestations was strongly related to the total duration of the EEG seizure manifestations and not to the duration of motor manifestations.